This has been an annoyance since I bought this rifle. That rounded nut backs out which causes the pivot bolt to back out. Next thing you know the barrel is too loose between the fork. I have had this gun apart numerous times which leads to worn fastener surfaces. Even new lock washers would not keep the assembly tight. I have two new bolt assembly's in my spare parts box, but wanted to give this repair a try first.
Started out with a 4-40 in mind but that little set screw hex stripped easy. Went to a 5-40 size with a stronger resistance to stripping. Screwed the nut onto an M7 bottom tap while working it. Put nail polish on the threads for hopefully keeping that set screw tight. With assembly properly tight I marked the bolt for a flat, and put a matching flat, Dremel ground, on the M7 bolt end. I bet this will work keeping the assembly tight.
Started out with a 4-40 in mind but that little set screw hex stripped easy. Went to a 5-40 size with a stronger resistance to stripping. Screwed the nut onto an M7 bottom tap while working it. Put nail polish on the threads for hopefully keeping that set screw tight. With assembly properly tight I marked the bolt for a flat, and put a matching flat, Dremel ground, on the M7 bolt end. I bet this will work keeping the assembly tight.